1. Field of the Invention
The present invention relates to a moving image shooting apparatus, a method of controlling the same, and a storage medium, and more particularly to a technique for preventing image disturbance from occurring when a frame rate is changed during moving image shooting.
2. Description of the Related Art
In the case of photographing an object moving vigorously e.g. in a sport, as a moving image, it is desirable to perform shooting at a higher frame rate than usual so as to improve the quality of the moving image.
However, the amount of data of a moving image captured at a high frame rate is large with respect to recording time, and hence shootable time over which shooting can be performed, with respect to the capacity of a storage medium, is reduced. Further, moving image shooting performed at a high frame rate requires high-speed operation of a circuit including an image pickup device, which causes battery to be steeply consumed, which results in reduction of shootable time.
As a solution to the above problem, there has been proposed a method in which only a specific scene is captured at a high frame rate during shooting. On the other hand, there has also been proposed a method in which when an object moving gently is to be shot as a moving image, shooting is performed at a lower frame rate than usual to thereby prolong shootable time. For example, in Japanese Patent Laid-Open Publication No. 2008-99110, there has been proposed a technique that makes it possible to maintain balance between the quality of a moving image and shootable time by changeably controlling the frame rate according to the motion of an object during shooting. Further, in Japanese Patent Laid-Open Publication No. 2009-141834, there has been proposed a technique for reducing luminance change, in particular, out of changes in moving image quality which occurs across a frame rate change.
In the above-described conventional techniques, when the frame rate is changed during shooting, the exposure time of an image pickup device changes, which causes a difference in the luminance level of an image before and after the frame rate change. This luminance level difference is adjusted by automatic exposure (AE) control, but there is a problem of low responsiveness, since AE control involves mechanical operation. More specifically, the adjustment of the luminance level difference by AE control takes time corresponding to at least several frames, and hence there is a fear that the viewer perceives the adjustment as temporary image disturbance. As a solution to this problem, it can be envisaged to adjust the luminance level by analog gain adjustment.
However, gain is increased e.g. for shooting a dark object, causing deterioration of the S/N ratio, and again, there is a fear that the adjustment can also be perceived as image disturbance. Therefore, this solution is not effective enough.